style: Improve #[derive(Parse)].

I want to do this so that I can get rid of Either<>. The reasons for getting rid
of either are multiple:

 * It doesn't generate as nice C++ code using cbindgen.
 * It isn't that nice to use either from Rust.
 * cbindgen has bugs with zero-sized types.

I started using this for ColorOrAuto and a few others, for now.

Differential Revision: https://phabricator.services.mozilla.com/D19844

components/style/values/mod.rs

@@ -125,6 +125,7 @@ impl Parse for Impossible {
     Copy,
     MallocSizeOf,
     PartialEq,
+    Parse,
     SpecifiedValueInfo,
     ToAnimatedValue,
     ToAnimatedZero,
@@ -147,19 +148,6 @@ impl<A: Debug, B: Debug> Debug for Either<A, B> {
     }
 }
 
-impl<A: Parse, B: Parse> Parse for Either<A, B> {
-    fn parse<'i, 't>(
-        context: &ParserContext,
-        input: &mut Parser<'i, 't>,
-    ) -> Result<Either<A, B>, ParseError<'i>> {
-        if let Ok(v) = input.try(|i| A::parse(context, i)) {
-            Ok(Either::First(v))
-        } else {
-            B::parse(context, input).map(Either::Second)
-        }
-    }
-}
-
 /// <https://drafts.csswg.org/css-values-4/#custom-idents>
 #[derive(Clone, Debug, Eq, Hash, MallocSizeOf, PartialEq, SpecifiedValueInfo, ToComputedValue)]
 pub struct CustomIdent(pub Atom);